1. Field of the Invention
This invention relates to an electromagnetic field intensity measuring apparatus, and more particularly to such an apparatus for optically measuring the intensity of an electric field.
2. Description of the Related Art
Electric waves are currently widely used in a variety of fields. Especially, today's computerization trend increases more and more the importance of electric waves as information transmitting media for broadcasting and communications. In the other fields, electric waves are prevailing widely as energy media e.g. in semiconductor manufacturing apparatuses, plasma heating process, etc. At present, it is necessary to understand accurately the detail of an electric field in order to develop the technology making use of it. Meanwhile damages to various electronic equipments caused by electric waves is being outstanding as a serious problem. To take measures against electric interference, abbreviated to EMI, it is necessary to accurately analyze the state of the electric field.
Much progress has therefore been made in the development and practical use of various kinds of electric intensity measuring apparatuses. In such a typical conventional apparatus, a sensor is installed in an electric field measuring place for measuring the intensity of an electric field. The sensor should be installed so as not to disturb an electric field to be measured and should be small in size so that the electric field can be measured in a small space.
However, the conventional electric field intensity measuring apparatus equipped with the sensor does not satisfy this demand. Further it is impossible to accurately measure the intensity of a low-frequency electric field by using a small-size sensor, especially such as commercial cycles.
In one example of the conventional electric field intensity measuring apparatus, a probe antenna serving as a sensor is installed in the electric field place, and the obtained electrical signal is transmitted to a detector circuit, which is installed outside the electric field to be measured, via a metallic conductive cable.
However, a metallic cable prevents not only freely moving and installing the probe antenna, but also measuring accurately the intensity of the electric field by disturbance.
In an effort to solve this problem, an electric field intensity measuring apparatus using electro-optic crystal such as LiNbO.sub.3 has been proposed.
One example of the electric field intensity measuring apparatus is shown in FIG. 8. This conventional apparatus comprises a sensor 10 disposed in an electric field measuring place 100, a light source 12 disposed outside of the measuring place 100, and a photo detector 14. The sensor 10 is optically connected with the light source 12 and the photo detector 14 via an up-link optical fiber 16 for inputting measuring light and a down-link optical fiber 18 for outputting modulated light, respectively.
The sensor 10 includes a polarizer 20, an electro-optic crystal 22, an analyzer 24, probe antennas 26a, 26b, and a pair of electrodes 28a, 28b mounted on opposite side surfaces of the electro-optic crystal 22. The electrodes 28a, 28b are connected to the antennas 26a, 26b, respectively.
When measuring an electric field with the conventional apparatus, measuring light is outputted from the light source 12 to the sensor 10 via the optical fiber 16.
The electro-optic crystal 22 constituting the main part of the sensor 10 is cut along the X axis, Y axis and Z axis, as shown in FIG. 8, so that the measuring light from the light source 12 are propagated in the direction of the X axis as a linear polarization light wave inclined by 45.degree. with respect to the Y axis by the polarizer 20. Therefore the light wave incident on the electro-optic crystal 22 via the polarizer 20 is decomposed into normal light (Y axis) and abnormal light (Z axis), which are then propagated independently of one another.
At that time the electric field detected by the antennas 26a, 26b is applied between the electrodes 28a, 28b as a potential difference. By an electro-optic effect of the crystal 22 resulting from this potential difference, the refractive index with respect to the abnormal light varies to cause a phase difference between the two light wave components, i.e. normal light and abnormal light, passed through the electro-optic crystal 22. This phase difference is then detected by the analyzer 24 disposed perpendicularly to the polarizer 20. Namely, when there is no phase difference, the measuring light will remain its initial linear state so that the quantity of light passing the analyzer 24 is zero. However, when a phase difference is caused due to the electric field, the light wave will be elliptically polarized to generate a light wave component to pass the analyzer 24. Since the quantity of light passing the analyzer 24 is determined by the applied potential difference, it is possible to measure the potential difference applied to the electro-optic crystal 22, i.e. the intensity of the electric field, by directing the light, which passed the analyzer 24, to the photo detector 14 via the optical fiber 18 and then measuring the amount of the light passed the analyzer 24.
In this conventional electric field intensity measuring apparatus, since the sensor 10 and the optical fibers 16, 18 are chiefly made of dielectric, it is possible to measure the intensity of an electric field accurately, with substantially no disturbance with the electric field in the electric field measuring place 100.
However, this conventional apparatus has a problem that the antennas 26a, 26b of the sensor 10 cannot detect the intensity of low-frequency electric field. If normal dipole antennas are used as the antennas 26a, 26b, it requires a huge length in order to detect a low-frequency electric field, thus making the sensor too large in size and hence not practical. Thus with the small sensor 10 of the conventional apparatus, it is impossible to measure the intensity of a low-frequency electric field, especially the intensity of an electric field near commercial cycles (50 Hz, 60 Hz).
Further, since the bulk crystal 22 is used as the main part of the sensor 10, the conventional apparatus is remarkably low in sensitivity of measuring an electric field so that it is difficult to measure the intensity of a very weak electric field. Yet with an ultra-small crystal 22 of 1 mm squares, a voltage of 300 V is required in order to change the phase of light passing such crystal 22 by 180.degree.. Therefore it is very difficult to measure the intensity of a very weak electric field, depending on the gain of the antennas 26a, 26b.